214 Ground improvement by deep vibratory methods
In a majority of contracts, the necessary ground improvement works will
be carried out by specialist contractors working as subcontractors. With
both types of contracts, the geotechnical investigation and its competent
interpretation by the engineer (either working on behalf of the employer
or for the turnkey contractor) is essential for a successful execution of the
foundation works. Regardless of who ultimately will bear the risk for the
subsoil conditions, it is in the best interest of geotechnical engineers to help
to make specialist foundation works such as ground improvement by deep
vibratory compaction a technically and scientifically accepted method.
Contract conditions will also contain a project description including a
time schedule to allow the specialist contractor to define the required plant
and equipment. A bill of quantities with payment and legal conditions are
the key elements of the contract which is generally complemented by technical specifications. The rules of the contract have to reflect the employer’s
right to have all works done according to the current state of the art.
The increasing importance of ground improvement in general, and the
rising demand for the deep vibratory methods of vibro compaction and
vibro replacement stone columns in particular, prompted engineering societies in many countries to work out standards, specifications, and notes of
guidance to support good practice in this specialized field of construction
industry (ICE, 1987; BRE, 2000; CFMS, 2005 or codes such as DIN EN
14731). In the United States, preference is given to case-by-case specifications that often accommodate the needs of the specific project in a better
way. These and the guideline specifications generally reflect experts’ knowledge in the technical field as well as in contractual affairs and serve for both
design-and-build and for measured contracts.
The previous chapters have highlighted the importance of a comprehensive soil report which is prerequisite for the ground improvement design.
It should contain all information on the soil which is generally necessary
for bearing capacity and settlement calculations under the loading conditions of the project. When vibro compaction of loose granular soils is
envisaged, the soil investigation has to include, apart from general information on in-situ densities and water table, stratification, sieve analyses, and
mineralogical composition of the sand layers. We have seen that unsuitable
cohesive soils are not just bypassed with the vibro stone column method.
Their strength and deformation characteristics are essential information
for the design. Equally important are the contractual regulations to cope
with nonconformance situations when assessing the effectiveness of the
ground improvement method. Whenever the deviations from the performance requirements are not caused by the quality of the works, specialist
contractor and engineer together have to work out a solution to remedy
this situation which will have to rely on an accurate description and of soil
properties and their meaningful interpretation.
On vibro compaction contracts, performance checks are normally
carried out by comparison of pre- and post compaction in-situ density
In a majority of contracts, the necessary ground improvement works will
be carried out by specialist contractors working as subcontractors. With
both types of contracts, the geotechnical investigation and its competent
interpretation by the engineer (either working on behalf of the employer
or for the turnkey contractor) is essential for a successful execution of the
foundation works. Regardless of who ultimately will bear the risk for the
subsoil conditions, it is in the best interest of geotechnical engineers to help
to make specialist foundation works such as ground improvement by deep
vibratory compaction a technically and scientifically accepted method.
Contract conditions will also contain a project description including a
time schedule to allow the specialist contractor to define the required plant
and equipment. A bill of quantities with payment and legal conditions are
the key elements of the contract which is generally complemented by technical specifications. The rules of the contract have to reflect the employer’s
right to have all works done according to the current state of the art.
The increasing importance of ground improvement in general, and the
rising demand for the deep vibratory methods of vibro compaction and
vibro replacement stone columns in particular, prompted engineering societies in many countries to work out standards, specifications, and notes of
guidance to support good practice in this specialized field of construction
industry (ICE, 1987; BRE, 2000; CFMS, 2005 or codes such as DIN EN
14731). In the United States, preference is given to case-by-case specifications that often accommodate the needs of the specific project in a better
way. These and the guideline specifications generally reflect experts’ knowledge in the technical field as well as in contractual affairs and serve for both
design-and-build and for measured contracts.
The previous chapters have highlighted the importance of a comprehensive soil report which is prerequisite for the ground improvement design.
It should contain all information on the soil which is generally necessary
for bearing capacity and settlement calculations under the loading conditions of the project. When vibro compaction of loose granular soils is
envisaged, the soil investigation has to include, apart from general information on in-situ densities and water table, stratification, sieve analyses, and
mineralogical composition of the sand layers. We have seen that unsuitable
cohesive soils are not just bypassed with the vibro stone column method.
Their strength and deformation characteristics are essential information
for the design. Equally important are the contractual regulations to cope
with nonconformance situations when assessing the effectiveness of the
ground improvement method. Whenever the deviations from the performance requirements are not caused by the quality of the works, specialist
contractor and engineer together have to work out a solution to remedy
this situation which will have to rely on an accurate description and of soil
properties and their meaningful interpretation.
On vibro compaction contracts, performance checks are normally
carried out by comparison of pre- and post compaction in-situ density